The invention in particular relates to a method and moulding system for manufacturing blade shell parts of a wind turbine blade, advantageously blades having a pre-bent curvature. Such blades are typically assembled from blade shell halves. One of the shell halves forms the pressure side of the blade and other shell half forms the suction side. The pressure side of the blade is also called the upwind side, as it faces the wind during operation of the wind turbine. The suction side of the blade is also called the downwind side, as it faces away from the wind during operation of the wind turbine.
By pre-bent blade is to be understood a blade which as seen from the root region towards the tip region at a distance from the root region extends forwards towards the wind in a forwardly curving manner so that the tip of the blade is positioned in front of the centre line of the root region. An example of a pre-bent wind turbine blade is disclosed in EP 1019631B1.
Wind turbine blades made of a composite material, such as fibre-reinforced resin, are flexible and when subjected to a gust they may flex up to about 6-8 m or more at the tip depending on the length thereof.
Pre-bent blades may have such a curvature that the tip is arranged 0.5-4 m or more from the centre line of root depending on the length of the blade.
As wind turbine blades have become progressively longer in the course of time and today can be more than 70 m long, it has become increasingly attractive to use pre-bent blades for wind turbines, as they allow for placing the wind turbine rotor close to the tower of the wind turbine, while still avoiding that the blade collides with the tower when subjected to a gust.
When manufacturing blade shell halves the mould is generally arranged so that the line corresponding to the centre line of the root region of the blade is substantially horizontal. This centre line often corresponds to a pitch axis of the blade.
However, when manufacturing the shell halves for pre-bent blades, especially blades pre-bent to arrange the tip about 1 m or more in front of the centre line of the root region, the fibre/resin ratio tends to be higher in the highest positioned areas of the mould surface than in the lowest positioned areas of the mould surface as seen in the longitudinal direction of the mould. This is especially the case in a longitudinally extending zone located at the lowest area of the mould as seen in the transverse direction thereof. In the above zone a plurality of fibre layers is frequently placed on top of each other in order to form a load-bearing structure of the blade shell half comprising a substantially higher number of fibre layers than laterally adjacent areas of the fibre lay-up.
Thus, due to the gravity acting on the resin, a too high fibre/resin ratio tends to be formed at the highest positioned areas of the mould and a too low fibre/resin ratio tends to be formed at the lowest positioned areas of the mould as seen in the longitudinal direction thereof.
A too high fibre/resin ratio has a negative influence on the fatigue strength of a composite material comprising fibre-reinforced resin. This is especially a problem when moulding the upwind shell half, as the highest positioned area of the mould surface is in the mid portion of the half shell when seen in the longitudinal direction and as the mid portion of the blade is subjected to high loads during operation of the wind turbine.
EP 2 404 743 by the present applicant discloses a method to ameliorate the above problem by dividing resin infusion into separate areas so as to mimisise the effect of gravity acting on the resin with the effect that the fibre/resin ratio can more accurately controlled of the finished wind turbine blade shell in the separate areas. However, even in this manufacturing setup, it has proven difficult to control the fibre/resin ratio of the separate areas of the wind turbine blade shell to a high degree.
GB 2 403 927 A discloses a method and apparatus for moulding composite articles. The method and apparatus comprises the use of a skin, which defines a mould cavity. A pressure sensor is connected to the skin so as to generate a signal indicative of pressure in the cavity. A control unit controls injection of resin into the cavity and may cut off the resin supply, if the sensed pressure becomes too high.